1. Field of the Invention
The present invention relates to a single tier drying section apparatus for drying a web of paper.
More specifically, the present invention relates to a vacuum roll for a single tier drying section.
2. Information Disclosure Statement
The introduction of the Bel-Champ.TM. single tier drying section at the Quinnesec Paper Machine No. 41 of Champion International was a revolutionary concept in paper drying technology.
Bel-Champ is a common law mark of Beloit Corporation covering the aforementioned single tier drying arrangement in which alternate sides of the web are dried during passage of the web through subsequent alternating top and bottom felted dryer sections. The Bel-Champ.TM. concept enhanced runnability while restraining the web against cross-machine directional shrinkage thereof.
In another dryer section of the Bel-Champ.TM. type designed by Beloit Corporation and installed at Ashdown Paper Machine No. 64 for Nekoosa Paper, Inc., the 7 single tier drying sections include a total of 52 vacuum felt rolls, each having a diameter of 26 inches and a shell length of 398 inches.
Each of the aforementioned vacuum rolls includes an outer rotatable shell which defines as many as 1 million holes drilled therein. Additionally, each vacuum roll includes a stationary inner shell, which is also drilled and which includes stationary isolating seals extending between the inner stationary shell and the outer rotating shell.
In order to draw the web towards the felt as the web extends around such vacuum roll, relatively complex internal damper mechanisms are utilized in order to direct and locate the partial vacuum within the vacuum roll in the vicinity of that portion of the vacuum roll being wrapped by the web and felt.
The aforementioned vacuum roll arrangements are complex, and a typical vacuum roll may cost in the region of $100,000.
The present invention overcomes the aforementioned problem by providing a greatly simplified vacuum roll arrangement in which a stationary vane or fin mechanism is located within a rotatable roll shell. The arrangement is such that when the roll shell is rotated by movement of the felt, a clearance exists between the vane or fin without the need for any costly seal arrangement therebetween.
The purpose of the stationary vane(s) or fin(s) mechanism is to prevent the air encompassed by the vacuum roll from rotating with the roll at the roll velocity. Testing has shown that without the aforementioned mechanism, the rotating air encompassed by the roll provides an effective barrier to air entering the vacuum roll. The shear force created by the internal boundary layer of rotating air is perpendicular to the direction of the air trying to enter through the drilled face of the vacuum roll shell. This impediment to air entering the roll prevents the air flow from attracting, and attaching, the paper web to the dryer fabric as it passes over the vacuum roll, the major purposes of this roll.
Flow into the roll can be controlled with a combination of partial vacuum at the roll journal and the clearance between the tips of the fin(s) or vane(s) and the inner surface of the shell. Once a partial vacuum level sufficient to cause flow into the roll has been established at the roll journal, there are two methods of changing the rate of flow:
1) Increasing the partial vacuum level at the roll journal alone will increase the air flow. Decreasing the partial vacuum level alone with have the opposite effect.
2) Increasing the length of the gap between the vane or fin tips and the inner surface of the roll shell will decrease the air flow rate. This is due to the increased depth of the boundary layer of air traveling with the roll shell causing a greater impediment to the air entering the roll through the shell holes. Decreasing the gap from the vane or fin tips to the inner surface of the roll shell has the opposite effect. The impediment is reduced with the thinner boundary layer of air.
Therefore, it is a primary objective of the present invention to overcome the aforementioned inadequacies and problems associated with the prior art arrangements and to provide a vacuum roll for a single tier drying section apparatus which makes a considerable contribution to the art of drying a web of paper.
Another object of the present invention is the provision of a single tier drying section apparatus which includes a vacuum roll having a rotatable roll shell and vane means disposed within a chamber defined by the roll shell, without the need for seals or the relatively costly machining of the internal surface of the shell.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter, taken in conjunction with the annexed claims.